1. Technical Field
This invention relates to a method of driving a display panel.
2. Description of Related Art
Recently, attentions are drawn to the plasma display panel (hereinafter, referred to as a PDP) having a plurality of discharge cells arranged in a matrix form, as a two-dimensional image display panel. Furthermore, there is known a sub-field technique as a driving method for such a PDP to display an image corresponding to an input video signal. In the sub-field technique, the display period in one field is segmented into a plurality of sub-fields so that discharge emission can take place every sub-field selectively on each discharge cell according to the luminance level as represented by the input video signal. This provides a visual perception at an intermediate luminance corresponding to the total emission period of within the 1-field period.
FIG. 1 shows an example of emission drive sequence based on the sub-field technique. See FIG. 14 of Japanse Patent Kokai No. 2000-227778 (patent document 1), for example.
In the emission drive sequence shown in FIG. 1, one-field period is divided into fourteen sub-field of SF1-SF14. All the PDP discharge cells are initialized to on-mode (Rc) only in the head sub-field SF1 of among those SF1-SF14. Meanwhile, in each sub-field SF1-SF14, the discharge cell is set to off-mode (Wc) according to the input video signal, thereby causing a discharge emission (Ic) only on the on-mode discharge cell over a period assigned to this sub-field.
FIG. 2 shows an example of emission drive patterns of within a 1-field period as to the discharge cell under driven based on the emission drive sequence (see FIG. 28 of patent document 1, for example).
According to the emission patterns shown in FIG. 2, the discharge cell initialized to on-mode in the head sub-field SF1 is set to off-mode in any one of sub-fields SF1-SF14 without having an occasion for return to the on-mode, as shown by the black circles. Accordingly, before set to off-mode, discharge emission takes place on the discharge cell in the successive sub-fields, as shown by the white circles. In this case, because the emissions in 15 patterns shown in FIG. 2 are mutually different in the total emission period within one field period, expression is available with 15 patterns of intermediate intensities. Namely, display can be with (N+1) gray-scale levels (N: the number of sub-fields) of intermediate intensities.
However, in this driving method, there encounters a problem of insufficient levels in gray scale because of the limitation in the number of sub-fields obtainable by dividing one field. For this reason, levels-increasing processing, such as dithering, is performed on the input video signal in order to supplement such insufficient gray-scale levels (see, FIG. 24 of patent document 1, for example).
In dithering, a plurality of pixels adjacent vertically and horizontally of the screen are first taken as one pixel unit, to respectively assign and add dither values, as mutually different coefficient values, to those of pixel data corresponding to the pixels of within the one pixel unit (data representing, with k-bit data, a luminance level represented by the input video signal). Then, the higher-order bit group is extracted out of the dither-added pixel data. In accordance with the higher-order bit group, driving is carried out as to any one of emissions in 15 patterns as shown in FIG. 2. Namely, emission is caused based on the weighting of different intensities provided to the pixels of one pixel unit. This allows for visual perception at the luminance corresponding to the mean luminance over the pixels of within one pixel unit.
Incidentally, it can be considered to apply, together with such dithering, what is called line dithering for emission-driving the pixels belonging to the display lines based on a display line group having a plurality of adjacent display lines by providing the weighting of intensities to the display lines of within the display line group. In such line dithering, the sub-fields for successive discharge emissions are provided different, in the number as shown by the white circles in FIG. 2, between the adjacent ones of display lines. However, because the sub-fields for successively setting the discharge cell to on-mode decrease in the number as the luminance level lowers for expression, line dithering is practically made impossible to carry out. Accordingly, it is not a practice to carry out a line dithering with a gray-scale level for expression at a luminance lower than a predetermined luminance. In this case, the luminance difference between gray-scale levels (with line dithering) for expression with a luminance higher than a predetermined luminance is provided nearly equal over the discharge cells belonging to each of the display lines.
However, the luminance difference differs from display line to display line, between a gray-scale level of expression at a lower luminance than the predetermined luminance (with no line dithering) and a gray-scale level of expression at a higher luminance than that gray-scale level (with line dithering). Accordingly, the luminance difference between gray-scale levels varies between the display lines, thus causing display noise and hence a problem of incurring image deterioration.